In the present description, the words path or channel are used interchangeably to denote a channel for propagating and transmitting the data. These channels will be able to be accumulated to provide wideband communication.
HF links notably provide a capacity beyond the line of sight or BLOS that allows long or even very long haul communications to be implemented without requiring the use of a satellite.
The technical context of the present invention relates more particularly to the use of high speed HF links (for example bit rates>19.2 kb/s) as dealt with in the patent application filed by the applicant under the number FR 10/04650, which proposes considering the use of a plurality n of conventional channels or paths with a typical width of 3 kHz of passband.
The long haul communication capability (for BLOS) of the HF links is reliant on the reflection of the HF waves (which typically range from 2 to 30 MHz) on the layers of the ionosphere, layers that have qualities that are not stable in time and space, which leads to strong variations in the propagation channel. To this instability of the channel is also added the ever-possible presence of various intentional or unintentional sources of scrambling, particularly at night when the transmitting portion of the HF spectrum is not as great.
Despite its instability, this channel has the benefit of transmitting long haul communications without it being first of all necessary to deploy a complicated or expensive infrastructure, in contrast to satellite communications, for example. If its better stealth is likewise considered, this explains why professionals seek to increase the bit rates provided on the HF links. A solution has been proposed in the aforementioned patent application for considering the use of a plurality n of 3 kHz channels, which may or may not be contiguous, so as to go further and provide higher useful bit rates for users of the HF band.
The modulation and coding schemes used in some standards are of monocarrier modulation type using a modulation and coding scheme that is reliant on a given constellation, for example PSK (phase shift keying) or QAM (Quadrature Amplitude Modulation) modulation, and a given correction code, for example a convolutional code, which may or may not be punctured, defining a useful bit rate. In the case of extension with a lower side band or BLI, two paths share the same digital modulation, with an even bit/odd bit distribution on one or other path, and hence the same useful bit rate, the correction code being common.
It thus appears that when there are a plurality of available transmitting channels distributed over a relatively wide band, for example of 200 kHz, and wideband transmission allowing high bit rates, greater than 32 kb/s, to be reached is performed, the various paths distributed over this wide band will not be able to see the same imperfections in the propagation channel. Typically, fading will be different, and intentional or unintentional scrambling will differ.
The use of a single modulation and coding scheme, i.e. one and the same constellation and an identical or shared correction code, will not allow the best adaptation of capabilities of the transmission channel. In order to succeed in transmitting on a path that has a high level of fading, one solution involves reducing the useful bit rate on all of the paths, including those that do not have high levels of fading.
Moreover, the generalization of an approach such as the one reserved for BLI therefore does not allow effective combat of the loss of a path. The reason is that, for BLI, the sudden degradation of a path causes the system to fall back to communication in a mode with a single useful band (called upper side band mode BLU), which creates the problem of a strong reduction in bit rate.
The solutions that exist in the prior art or the immediate declension of said solutions in the case of n paths are of two types that are summarized below.
A first solution, shown schematically in FIG. 1 in the case in which the use of two channels simultaneously is considered, involves processing the bands separately, each one typically being provided with a dedicated modem implementing the current 3 kHz or 6 kHz standard, with a change to wideband processing solely at the level of the radio, by means of summation of the various carriers. The odd bits will be processed via the channel Ch1, chain 101, and the even bits via the channel Ch0, chain 102. Equally, at reception, two processing paths 103 and 104 will be used to process the even and odd data bits. This type of solution will not allow there to be a diversity gain, since each path will in fact be processed separately. In the case in which a single path is under consideration, with an approach of mono-carrier type, a single modulation and coding scheme is used, and it is management of link establishment/link maintenance (ALE/ALM) type that can introduce a dynamic character. The waveform under consideration is generally autobaud, which means that the waveform includes a specific transmission capability, commonly called separately decodable and demodulable autobaud field, which indicates the modulation and coding scheme used for the rest of the frame (or up until the next autobaud field), the change of bit rates being remotely controlled by the upper layers, typically by an ARQ controller.
In the description that follows, the processing chain being known to a person skilled in the art, the designations in the figures will be as follows:
for the transmission chain in FIGS. 1, 2 and 3, FEC: the error correction code, I: the interleaving, SYM: the symbol formation, FR: the framing step, M: the modulation step; the step SC in the diagram for scrambling.
for the reception chain in FIGS. 1, 2 and 3: g(t) the filtering, SYN: the synchronization, BDFE: the frame equalization step, SYNP: the prediction of the synchronization, DI: the deinterleaving, D: the decoding of the data.
This first type of solution (with coding and modulation processing separated on a path-by-path basis) does not allow full advantage to be drawn from the fact of using parallel paths, because such an approach does not bring about any diversity gain. It is moreover one of the reasons for which the BLI solution, combining two channels, as proposed in the MIL118-110B standard that is known to a person skilled in the art, introduces coding diversity by pooling the error correction coding and interleaving stage. However, this solution does not ensure the capability of easily operating with a blocked path (that is to say that the path is not transmitting, either on account of the propagation itself or because the channel is occupied by an intentional or unintentional source of scrambling) when the interleaving, bit rate, etc., or even number-of-paths information is variable because this information that is required for correctly decoding the frame is shared between the various paths. In the absence of appropriate signaling (typically in order to establish what has been lost), the result in the (conventional) case in which two paths are used is that the data transmitted on the two channels are lost when one is blocked because the standard approach uses the two paths in coupled fashion, by using the autobaud fields of the two paths to define the same and single modulation and coding scheme to be used on these two paths (the even bits traveling on channel 0 and the odd bits on channel 1).
A second solution, shown schematically in FIG. 2, involves extending the principle of the BLI by pooling the FEC correction code 205 and the interleaver 206 with a plurality of paths Chi, at the processing chains 201, 202, in order to provide a coding diversity gain, and by using the same modulation parameters on the various modem paths, before the frequency transposition 209 and likewise the summation 210 of the various carriers in the wideband radio. As mentioned above, this solution will not allow adaptation to the propagation condition differences of each of the channels under consideration. On reception, the pooling will be found for all the paths Ch of the deinterleaver 207 and the decoding 208. This second type of solution (extension of the BLI solution) which involves the use of the same modulation on each of the paths by sharing the correction code and the interleaver, will therefore allow a coding diversity gain, contrary to the first solution but on the other hand will render the system sensitive to the loss of one of the paths, as in the BLI solution with two combined paths. The reason is that, in the BLI solution, it will be noticed that the sharing of the autobaud information on the various paths (for BLI, channel 0 transmits the bit rate information, and channel 1 transmits the interleaver used), which is the information that allows the demodulator to know the mode that is used, risks making it impossible to use the various paths when one of them is lost completely, for example in the case of scrambling. This point, which was already a problem in BLI mode, and which had therefore led to the implementation of a mechanism for returning to the BLU monopath case in the event of a problem, becomes problematical in a context in which there is a change to n paths, since the probability of having a blocked channel increases greatly. Such a mechanism would therefore become very unstable and inefficient a priori.
None of the two approaches that have been set out above allows efficient use of the n paths in parallel, and combination of at least the various following advantages: improvement of the transmission owing to coding diversity, minimal protection of the autobaud information against the risk of blocked channels, capability of having different bit rates on the various paths, on the basis of the quality of the propagation channel under consideration.
The document by M. Jorgenson et al. entitled “Meeting military requirements for increased data rates at HF”, MILCOM 2000, 21st Century Military communications Conference Proceedings 22-25 Oct. 2000, PISCATAWAY, N.J., USA, IEEE, XP010532080 discloses the use of independent modulations on a plurality (n) of channels in one and the same system, with the same modulation parameters for each channel, and a correction code that is common to all channels.
The document by S. Trinder et al. entitled “Optimisation of the stanag 5066 ARQ Protocol to support high data rate HF communications”, MILCOM 2001, Proceedings. Communications for Network-Centric Operations: creating the information Force. XP010579059 discloses a system in which the addition of redundancy allows the success of reception of the information to be optimized.
One of the problems posed is therefore that of having, for one and the same communication, efficient transmission on n paths in parallel, each of these paths seeing a potentially different propagation channel, while ensuring that the loss of one of the paths or of a plurality of these paths does not destroy the whole communication.